请问在英语中用在句子开头的现在分词,过去分词,动名词,以及动词原形有何区别?

现在分词放在句首表示动作是主动或正在进行或完成Having done his homework,he played basket-ball
过去分词放在句首表示动作是被动Praised by the neighbours,he became the pride of his parents.或者Given more time,I'll be able to do it better.或者Once seen,it can never be forgotten.
动名词放在句首表示把动词名词化Eating no junk food is a good habit.
动词原形放在句首的句子为祈使句Sit down,please.